1. Field of the invention
The present invention concerns a method and a device for illuminating a photosensitive material disposed upon a face plate for the formation of a cathodoluminescent screen of a perforated mask type color television tube.
2. Description of the prior art
A color television tube comprises a frontal face plate upon the internal surface of which is deposited the screen which is normally formed of vertical bands of cathodoluminescent material emitting, when it is bombarded by an electron beam produced by an electron gun, a red, green or blue color light. The screen thus comprises a succession of vertical three-band assemblies, each assembly presenting a red band, a green band and a blue band. Each color is excited by a corresponding electron beam. In a tube, often known as "matrix", two bands of adjacent phosphors are separated by a black band of graphite, which produces an image of improved contrast. In a mask tube, for selecting the colors, i.e. so that the beam intended for one color, for example blue, only strikes the phosphor of that color (blue), a perforated mask is provided in front of the screen, the position and the disposition of the slots of this mask ensuring the said selection.
Since the position of the mask with respect to the screen deposited on the face plate must be determined accurately, the mask is secured to the face plate of the tube prior to the formation of the screen in order that this mask can be utilized to produce the screen. Each of the luminescent substances is deposited in the following way: the internal face of the face plate is coated with a solution of this substance in a photosensitive material which hardens when it is illuminated by an ultra-violet radiation (UV), then through the mask this solution coating the internal face of the face plate is illuminated by an optical system that comprises an UV radiation source and an objective simulating the tube deflector. The position of the optical system, especially of the UV lamp, depends upon the color of the phosphor in solution. In this way, only the photosensitive material situated at the sites provided for the determined color is illuminated and can thus harden. The material situated at the other sites does not harden and does not adhere to the glass; it can be cleaned by washing with water or by means of any other liquid.
The UV source emitting a radiation the intensity of which is not constant in function of the emitting direction, between this source and the screen is disposed a filter which compensates this lack of uniformity so that the intensity of radiation reaching the screen is substantially constant on the surface of this latter; indeed, if it were not so, the surface area of the hardened sites would not be constant.
The mask is formed of slots disposed successively along vertical lines while it is desired to form continuous vertical lines upon the screen. If no particular measures are taken, discontinuous lines of phosphors and graphite will thus be obtained on the screen. In order to prevent this defect, during illumination, the illumination device is moved in a vertical direction.
For the manufacture of each type and of each dimension of color television tube, it is necessary to provide a particular illumination device. In other words, a device or apparatus designed for one determined type and one size cannot be used for another size or another type. Furthermore, the exposure times are relatively long.
The present invention overcomes these drawbacks.
The device according to the invention comprises: a luminous source emitting a pencil of light or fine luminous beam, preferably a laser, a deflector to deflect the beam so that it scans the surface of the face plate, a light modulator and a programmable apparatus, such as a computer, to control, on the one hand, the light modulator so that the luminous intensity received by the screen is practically constant, and on the other hand, the scanning to light up the vertical bands on the screen through the perforated holes of the mask.
The light modulator acts in a similar way to the filter of the illuminating device of the prior art, i.e. it is controlled so that in each point or dot of the screen, the quantity of luminous energy received per surface unit is constant. It is, however, to be noted that in theory the pencil of light has a constant luminosity whatever the direction whereas this is not the same with the known apparatus; but the compensation to be performed results from the fact that the intersection of the pencil of light by the screen is not the same from one point to another of this screen; this section is greater on the edges than at the center and thus, the energy density received per surface unit is lower on the edges than at the center, i.e. the light modulator is controlled so that the intensity is lower at the center than on the edges.
When the luminous source is constituted by a laser, the exposure time is small, thereby reducing the manufacturing duration.
In one embodiment, the programmable apparatus also controls the displacement of the center of deflection. This displacement, when it is performed parallel to the lines of the screen, allows to pass from the illumination of one band of determined color to a band of another color. A displacement of this center parallel to the vertical bands of the screen ensures the continuity of the bands of phosphors and possibly the black bands. Furthermore, a displacement of the center of deflection perpendicularly to the screen allows to adapt the illumination device to the manufacture of other types and dimensions of tubes.